Prior art mixed-compression inlets have been selected for application to supersonic propulsion systems for supersonic aircraft that cruise at Mach numbers higher than 2.0. For mixed-compression inlets, maintaining the terminal shock at the inlet throat station provides optimum internal performance. This operation provides high-pressure recovery and minimizes flow distortion at the engine face. These inlets, however, have a discontinuous airflow characteristic known as inlet unstart. For these conventional mixed-compression inlets that employ simple, basic fixed-exits on the inlet bleed systems, a small airflow transient can cause the terminal shock to be displaced forward of the throat position, where it is unstable and is abruptly expelled ahead of the inlet cowl. This shock expulsion or unstart causes a sharp reduction in mass flow and pressure recovery and a large drag increase. Inlet buzz, engine compressor stall, and/or combustor blowout may also occur. Obviously, an inlet unstart is extremely undesirable because of the adverse effects not only on the propulsion system itself, but also on the aerodynamics of the aircraft. If inlet unstart does occur, complex mechanical variations to alter the inlet geometry are required to re-establish the initial operating condition. Certification of an inlet that could unstart has been a concern since the U.S. Supersonic Commercial Transport (SST) program of the 1960's.
Both external airflow transients such as atmospheric turbulence and internal airflow changes such as a reduction in engine airflow demand can cause the inlet to unstart. For an internal airflow change, the inlet should provide a margin in corrected airflow below the value that provides optimum performance without incurring unstart. This margin is defined as the stable operating range. Conventional mixed-compression inlets may be designed to have a limited stable range that is provided by the capability of the performance bleed system to spill increased airflow as the terminal shock moves upstream in the throat region. However, with performance bleed exit areas that are fixed, this stable range may not be adequate to absorb many of the airflow transients that are encountered by a supersonic propulsion system. An increased stable margin is currently provided for these inlets by operating them supercritically with a resultant loss in performance. Since any loss in inlet efficiency is related directly as a loss in thrust of the propulsion system, supercritical operation is undesirable.
In the prior art, based on existing aircraft, inlet performance is compromised to obtain increased stability margin. These aircraft incorporate fixed exits on the bleed systems. Therefore, the inlets must be operated with increased bleed at the nominal operating condition and at a reduced total pressure recovery to maintain a sufficient operability margin. This compromised inlet operation significantly reduces the overall efficiency. Actual flight operation of a complex valve control system on an inlet stability bleed system has not been accomplished.
To provide the necessary inlet stability without compromising steady state performance, the inlet can be designed to allow the throat bleed to function as a throat stability bleed system. This system prevents inlet unstart by allowing the throat bleed to compensate naturally for changes in diffuser exit airflow demand. Past experimental research data on large scale inlet models have shown that large increases in bleed may be provided as the inlet operation proceeds from supercritical to minimum stable (just prior to inlet unstart) conditions, without prohibitive amounts of bleed during normal operation, if the bleed exit area can be controlled by a valve to maintain a near constant pressure in the throat bleed plenum (Sanders, Bobby W.; and Mitchell, Glenn A.: Throat Bypass Bleed Systems for Increasing the Stable Airflow Range of a Mach 2.50 Axisymmetric inlet with 40-Peecent Internal Contraction. NASA TM X-2779, May 1973). This area variation can be provided by an active control that senses shock position and regulates valve exit areas or by high-speed valves that react to bleed plenum pressure changes that occur when the terminal shock changes position.
In the prior art, based on experimental research testing, inlet stability-bleed airflow has been controlled during research studies by poppet valves, by vortex valves, and by research hardware that provided a variable exit area capability. Poppet valves, such as described in U.S. Pat. No. 3,799,475, when used with an inlet stability bleed system can provide very large stable operating margins. However, a requirement of a large volume inside the cowl is inherent in the application of this valve concept. In order to control and pass large amounts of airflow during transients in the inlet airflow, airflow must be exhausted around the periphery of the valve. A large internal cowl volume to house the valve and for ducting of the inlet flow to the valves may lead to high inlet external cowl angles and thus increased drag. Vortex valves (Sanders, Bobby W.; and Mitchell, Glenn A.: Increasing the Stable Operating Range of a Mach 2.5 Inlet. American Institute of Aeronautics and Astronautics, AIAA Paper 70-686. June 1970) can offer a more compact integration into the inlet cowling; however, the capability of vortex valves in providing increased inlet operability margin is limited. The vortex valves also require a high-pressure control airflow with an associated performance penalty. While the valves described above may be used for some applications, valves that can simultaneously control bleed from several bleed regions and allow the use of low profile external cowling to minimize drag are required. This type of valve is disclosed herein.